For headphone with less than 50ohm, you only need to replace two SMD resistor and two through-hole capacitors. For headphones with higher impedance, you may want to replace the opamp too.
I am really impressed with the end result of this modification. The geezer that deserves all the credit for this mod is the "Northwest Audio & Video Guy". If your into audio or analog electronics, then you'll love his blog. Well, it's more an encyclopediog, than a blog, going by the size of his articles. Great stuff. Here's the article describing this particular hack in the deepest possible detail:
An excerpt From the creator's blog:
"Behringer likely designed the UCA202's headphone jack for use with higher impedance studio headphones. While it can drive such phones fairly well, it's not very compatible with typical consumer 16 - 32 ohm headphones. (...) The new amp can much better drive lower impedances at low distortion and generate much more output power from the same power supply voltage. A few other component changes were also required to optimize the new design. (...) Using Ultimate Ear's headphones, the frequency response improved from +/- 7 dB to a much more neutral +/- 1 dB. (...) The modified Behringer UCA202 has enough power to satisfy most anyone (...) And the output impedance drops from a very poor 50 ohms to only 2.5 ohms. It retains its previously low distortion, similar noise levels, and most other characteristics."
- ESD protection gear (if you don't use it, you might fry your hardware)
- SMD soldering skills (mostly if you are going to replace the opamp)
- fine-tipped soldering iron
- flux (cheap resin-based flux is fine)
- optional: solder wick
- optional: (cheap) multi meter
- A vice or something else to hold the PCB while working on it
- Philips screwdriver
- Needle-nose pliers
- optional: fine wire cutter
- optional: 1x opamp AD8656 in SOIC package, costs about 3.30 euro a piece or 2.37 per 10 euro at Farnell
- 2x 6v3 / 2200uf (I used 1600uF, as that was what I had laying around.)
- 2x 2.2ohm / 1% 0806 SMD resistors
NOTE 1: The Behringer UCA 202 hardware seems to be 100% identical to that of the UCA 222. The only difference is the color of the housing.
NOTE 2: I used the SOIC package version of the OPAMP. The MSOP package may be better fit, but is cost 50% more than the SOIC package, so I just went with the SOIC.
NOTE 3: The original article mentions optionally using the possibly better performing opamp AD8397. I did not choose to try that option because it was more expensive, untested and the results of using the less expensive AD8656 are more than satisfactory for me.
Step 1: Open It Up
NOTE: At this point it would be wise to get your ESD protection gear out and use it.
Step 2: Optional: Desolder the Opamp
NOTE: The opamp types of the original opamps used in these soundcards are known to be either 4558 or 4556A. If your soundcard has another type, then you may consider not doing this modification. Unless ofcourse you know how to read datasheets.
WARNING: In my soundcard, the opamp was not only soldered, but also actually GLUED to the PCB. And very tightly too. This was recognizable by the tiny red blobs of dried glue on the PCB at the long ends of the opamp. I underestimated the strength of this glue: I was not able to pry the opamp off from the PCB while desoldering the legs. The result was that the PCB got so hot that one and a half pad delaminated from the PCB. I ended up cutting the opamp's legs to remove it from the PCB. And still, using pliers, I had to use a surprising amount of force to pull the opamp loose. Actually, you need to TWIST the chip of the board, because pulling might not even work. So if you see this red glue, or don't care about the original opamp: Go for desoldering method 1, or else you might damage the PCB.
That said, on we go... Desoldering can be done in a few ways:
Method 1. Cut the legs off from the opamp. Then use needle-nose pliers to lift the opamp from the PCB. After that, desolder the legs one by one.
Method 2. Heat the legs one by one and lift each leg up from the PCB with a pin. When all legs are up, use pliers to remove the opamp.
Method 3. Add some extra solder to four of the legs on one side. Melt the resulting blob and use pliers to lift that side of the opamp from the PCB. Now do the same on the other side.
Method 4. Heat all pins by 'scraping' past the pins quite fast in a repetitive fashion. Meanwhile try to lift that side of the opamp. When that side has been lifted, then do the other side.
Method 5. Using a hot-air desoldering device. I wouldn't do it, for fear of frying the rest of the components. I prefer the slower soldering iron in this case.
Step 3: Optional: Solder the New Opamp in Place
Optional but recommended: Clean the pads using some flux. This will make soldering the new opamp much easier. You might also want to scrape away some of the residual glue. But be very careful! You may end up damaging some tracks under the glue!
Now place the opamp and solder it in place. Note the orientation of the opamp: The little dot on the opamp has to be near R41 on the PCB. The opamp I used has a SOIC package, which is actually a bit smaller than the original IC, which appears to be an MSOP package.
NOTE: Note the copper wire connected to the top left pin. I had to do that because I destroyed the pad while desoldering the opamp. Not very aesthetic, but it works.
Also, bridge (shortcircuit) the two 47 ohm restors (R38 and R41) by placing a large-enough blob of solder on the resistors. Check all soldering joints for proper connection using a multimeter in conductivity testing-mode.
Step 4: Replace the Two Resistors
NOTE: In the example you can see each of the 47ohm resistor replaced by a stacked pair of 4.7ohm resistors in parallel. It was all I could manage at the time because I had no 2.2 ohm resistors laying around.
Step 5: Desolder Capacitors C14 and C15
Step 6: Optional: Lengthen the Legs of the Replacement Capacitors.
NOTE: Watch out for defective capacitors. These can usually be recognized by stuff leaking from the top or a bulging top. Many motherboards from broken PC's are 'broken' because the capacitors were of poor quality and died.
Step 7: Solder the Replacement Capacitors in Place
NOTE: Make sure you get the polarization of the capacitor right. You can see how it should be placed by looking at the silkscreen on the top of the PCB.
You will need to be bit creative with the placement of the larger caps to be able to fit them in the case. If you make the leads of the caps really long, then you can lay them horizontally on the PCB. Also, is they don't fit properly, you may need to cut a few bits off from the inside of the casing. With the 1600uF caps that I used, that was not necessary. Take a look at the images to get an idea how to place the capacitors.
Step 8: Test It and Re-assemble the Housing
Now, if all is well, then close up, clean up and enjoy the music and all of life's other wonders.